As a heterojunction bipolar transistor (HBT) in which a group III-V compound semiconductor is used, an InGaP/GaAs heterojunction bipolar transistor is widely used in which an emitter layer made of InGaP, which is a wide band gap semiconductor, is employed and other layers are made of GaAs in order to increase the current gain and the current injection efficiency.
In such an InGaP/GaAs heterojunction bipolar transistor, employing a base layer made of InGaAs having a smaller band gap than GaAs makes it possible to reduce a turn-on voltage (see, for example, Patent Literatures 1 through 3).